they told us most of how they traced the Finnish guy's monero. After arresting him, they sent an information request to a swap service which knew precisely what address they sent the monero to (because monero does not encrypt the recipient address), and then they sent another information request to binance, who knew that address showed up as a "ring member" in a transaction to their exchange. So they knew it was him. Didn't require breaking any cryptography. Just following the trail through monero's public ledger.